High voltage image tube



ug- 1, 1961 G. L.. KRIEGER ET AL 2,994,798

HIGH VOLTAGE IMAGE TUBE Filed Dec. 26, 1946 QQQWN. N gum. Q.m\ sonomww.. QQ, m MM5, .N GG MM A N\ WW .MN NN NN lm. @6 G G uw. m\

MN MN Gttomeg United States Patent O 2,994,798 HIGH VOLTAGE IMAGE TUBE Gardner L. Krieger, Princeton, NJ., and George A. Morton, Oak Ridge, Tenn., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Dec. 26, 1946, Ser. No. 718,505 Claims. (Cl. 313-65) This invention relates to image tube adapted to produce visible images from light and infra red energy, as in the application filed October 5, 1946, Serial No. 701,588, of Flory, Morton and Ruedy which is now U.S. Patent No. 2,506,018, issued May 2, 1950. In certain uses of these tubes, it is desirable to view a wide field in the dark. The tube of said application is designed for a narrow ield and when used to view a wide eld the conversion of incident energy to luminescence is lower than desirable. It might be thought that this could be remedied by applying much higher voltages to the auxiliary and main lenses in the tube but this is not practical because of arcing and cold emission.

It is an object of this invention to provide a tube construction that permits use of high anode voltages without inducing this action.

It is another object of this invention to increase the conversion in image tubes without altering the main aperture lens and the correcting lenses of prior art tubes.

Another object of the invention is to increase the volt velocity of the photoelectrons of an image tube without altering the voltages applied to the main lens.

Another object of the invention is to produce increased conversion in an image tube by the addition of a plurality of accelerating anodes between the main aperture lens and the phosphor screen and applying successively higher voltages thereto.

Other objects of the invention will appear in the following specication, reference being had to the drawing in which:

FIG. l is an axial section of an image tube embodying the invention; and

FIG. 2 is a diagrammatic illustration showing the potentials applied to the electrodes.

Referring to the drawing the tube comprises an envelope, say of glass, having a main portion 1 and a reduced end portion 2 joined by a wall or ange 3. To the end of the portion 2 is fused a hemispherical photolens 4, the purpose of which will be referred to later.

Anode 5 has an end flange supported on the end flange of anode 6 by a plurality of glass or other insulation joints 9 and the anode 6 is supported on anode 7 by a plurality of glass beads 10 and sealed-in wires. Anode 7 is similarly mounted on anode 8 by beads 11. Anodes 5 and 6 constitute the main lens and the anodes 7 and 8 are weak lenses and are referred to as auxiliary lenses. The photocathode 12 usually consisting of silver is formed as a thin transparent lm on the end of the tube and then caesium activated.

A plurality of spring snubbers 13 welded to anode 8 and resting at their curved free ends on the glass envelope 1 support the end of the lens assembly facing the cathode. Spring contact 14, shown in dotted lines, is soldered to wire 15 which is sealed into the wall 3. This contact engages the silver coating, or a metal film strip in contact with the photocathode so that negative potential may be applied thereto. The wire 15, of course, will be properly insulated from the anodes as by a ceramic covering 16. The lead wires for the anodes 6, 7 and 8 would be similarly insulated and would be sealed in the wall 3 but these are prior art construction and are not shown. Lead 17 is connected to anode 5 and is sealed through the wall i2,994,798 Patented Aug. l, 1961 ice 3. Lead 18 is connected to one end of an arcuate tungsten wire 19, the other end of which is connected to the end 'ange of anode 6. This tungsten wire may have a plurality of caesium beads 20 that are ashed by applying potential between lead 18 and the lead to anode 6, the latter not appearing in the drawing. After the caesium is dashed to sensitize the photocathode 12 in a known way the lead 18 is not further used.

The mid-portion of the anode assembly is supported by the leads passing through the wall 3.

As thus far described the construction of the image tube is substantially the same as that disclosed in said application of Flory, Morton and Ruedy. In our invention, we add three additional anodes 21, 22, 23 between the anode 5 and the phosphor screen 24 coated on the at side of hemispherical photolens 4. Each of these anodes is coaxial with, and of the same diameter as, anode 5. The anodes 21, 22, 23 have anges on their ends facing the phosphor screen that are sealed through the portion 2 of the glass envelope and their potentials are applied thereto. Without limiting the invention to particular dimensions it may be said the length of anode 5 is substantially three-quarters of an inch and its outside diameter is approximately live-eighths of an inch. This will also be the outside diameter of the additional anodes 21, 22 and 23. The three latter anodes are about one-half of an inch long and are spaced from each other, and anode 21 from the anode 5, by about an eighth of an inch.

Connection is made between the protruding flanges of anodes 21, 22 and 23 and the proper potential terminals of the voltage source.

It is desirable to produce an electron volt velocity at the screen 24 of 16,000 volts or more. This cannot be obtained merely by increasing the voltage of anodes 8, 7, `6 and 5 successively so that the anode 5 would have, say, 16,000 volts, because in caesiating the photocathode, caesium deposits on the anodes 8, 7 and 6 and increasing the voltage of these three anodes induces arcing and cold discharge which produces a high background in the picture on screen 24. We have found that the desired iinal high electron volt velocity may be obtained by the addition of the three anodes after the main lens 5, 6 without cold emission or arcing.

The additional anodes 21, 22 and 23 constitute a series of weak electron lenses which slightly increase the convergence of the system to bring the demagniiication down to as low as desired for increased brightness of the image. To bring the image back into focus on the screen 24 if required the voltage on anode cylinder 6 may be made slightly more positive. Increasing the potential of anode 6 also has the advantage of increasing the eld strength at the photocathode which improves the denition from the standpoint of chromatic aberration.

The magnification of the image tube in said co-pending application is about one-half to give a greater brightness of the image. In our improvement this brightness may be maintained or increased by reducing the magniiication to one-quarter or one-eighth. The hemispherical lens 4 magnies this image and an additional lens ordinarily used but not shown, further magnies the image. The three additional anodes introduce some spherical aberration but this is beneficial as it reduces the pin cushion eiect. They, however, tend to increase the curvature of the image but this is more than compensated for by the increased depth of focus due to the higher voltages used.

There is another advantage resulting from the addition of three anodes with increased voltages after the main lens. It enables one to use more etlicient phosphors such as zinc-cadmium-sulphide which has not been used in this type of tube because it is rendered substantially inert by action of the caesium when the photocathode is 3 activated. The use of the high voltages enables one to put a iilm 25 of aluminum 500 to 1000 A. thick over the phosphor screen to protect it from the caesium. The aluminum lm, by way of example, may be applied as follows:

The zinc-cadmium sulphide is settled onto the flat surface of the hemispherical lens 4 in a water suspension to which 1% by weight of potassium silicate is added. A drop or so of collodion is added to a cup of pure water to form a iilm about 500 A. thick. The lens 4 with its screen coating submerged in this is carefully lifted out of the water which deposits the collodion lm on the screen. The excess water is removed by lter paper and the screen is dried. A lm of aluminum 500 to 1000 A. thickness is then evaporated onto the collodion covering the phosphor screen. The photolens 4 is then sealed into the end 2 of the envelope as by radio frequency heating. The high voltages on the additional electrodes give the electrons suicient velocity to pass through the protecting aluminum coating to produce the photoimage with greatly increased brightness.

While certain specic embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without ,departing from the spirit and scope of the invention.

a photocathode at one end of the envelope, a fluorescent screen at the other end, an anode cylinder having one end facing the photocathode of substantially the same diameter as said photocathode and having another end of smaller diameter than said photocathode, an anode adjacent said other end of the rst anode of substantially its diameter, said anodes constituting the main electron lens of the tube, and a plurality of spaced accelerating anodes between said main electron lens and the fluorescent screen.

3. A cathode ray image tube comprising an envelope, a photocathode at one end of the envelope, a iluorescent screen at the other end, an anode cylinder having one end facing the photocathode of substantially the same diameter as said photocathode and having another end of smaller diameter than said photocathode, auxiliary anodes spaced apart between the rst mentioned anode and the photocathode, an anode adjacent said other end of the first anode of substantially its diameter, the rst and last mentioned anodes constituting the main electron lens of the tube, and a plurality of spaced accelerating anodes between said main electron lens and the fluorescent screen. v

4. A cathode ray image tube comprising an envelope, a photocathode at one end of the envelope, a fluorescent screen at the other end, an anode cylinder having one end facing the photocathode of substantially the same diameter as said photocathode and having another end of smaller diameter than said photocathode, an anode adjacent said other end of the first anode of substantially its diameter, said anodes constituting the main electron lens of the tube, a plurality of spaced accelerating anodes between said main electron lens and the fluorescent screen, and a hemispherical photolens adjacent said screen.

5. A cathode ray image tube comprising an envelope, a photocathode at one end of the envelope, a fluorescent screen at the other end, a main electron lens adapted to produce an electron image on said screen, a plurality of accelerating anodes between said main lens and said screen and an electron permeable ilm on said screen to isolate it from the space between the screen and the photocathode.

Kautz May 2, 1939 Morton Feb. 6, 1940 

